Therefore, my beloved, as you have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling; for it is God who works in you both to will and to do for His good pleasure.

But Paul is calling his beloved brethren in the Philippian church(literally; congregation), (and us too), to work out your own salvation with fear and trembling. This appears to contradict the clear, biblical teaching that salvation is by grace alone through faith alone in Christ alone. But Paul is not calling us to work for our salvation. We are not expected to save ourselves; to justify ourselves; to sanctify ourselves and glorify ourselves - for it is God who is working in us and God has promised to finish all the good work that He started in us, when we first trusted Christ as Saviour.

These verses are a call or command (imperative in the Greek) to every Christian to obey God. That includes pursuing holiness with all our hearts, and sacrificially submitting to Him. God wants us to be serious in our obedience. Yet God Our Father is the one who really helps us do all of this by His Holy Spirit. That is the message of Ephesians 5:17-18. Galatians 5:15-23 teaches us that the Holy Spirit is at work to transform us. God is working in us and every true Christian will want to be obedient (Philippians 1:6).